Generally, in a wind turbine generator, sensors each for measuring a load applied to a wind turbine blade are attached to a blade root part and the like, and a load is calculated by processing data measured by those sensors. However, since the relation between a load applied to each of the wind turbine blades and a distortion is not constant due to individual differences which occur at the time of manufacture of blades and at the time of attachment of the sensors, there is proposed a method of measuring a load for each wind turbine blade and calibrating a load value.    Patent Citation 1: U.S. Pat. No. 6,940,186